1. Field of the Invention
The invention pertains to apparatus and methods to treat polymeric materials, and more particularly to systems and methods for plasma assisted oxidation and stabilization of thermoplastic materials in preparation for high-temperature carbonization.
2. Description of Related Art
The production of carbon fibers from thermoplastic-based fibers, e.g., polyacrylonitrile (PAN), is typically a multi-step process in which the PAN fibers are first treated in an oxidizing atmosphere at temperatures ranging from ambient up to about 250-300° C., while the fibers are maintained under tension inside of large furnaces. The oxidized fibers are subsequently carbonized at temperatures up to about 1000-1200° C. in an inert atmosphere (carbonization step) and then further heated up to about 3000° C. (graphitization step). Traditionally, the first oxidation or stabilization processing step is the most time-consuming and rate-limiting step in conventional carbon fiber manufacturing, and it requires more space than all other steps combined.
Several methods for PAN-precursor stabilization are known. The most common method is stabilization in air, which usually requires hours to achieve full stabilization. Here the PAN-precursor may be heated in air at a carefully controlled rate up to 250° C. either in a batch process or by heating the precursor tow continuously as it is transported through a furnace or kiln, which contains several temperature zones. Much development work has been directed toward the goal of modifying the conventional stabilization step and reducing the processing time. For example, numerous publications indicate that the stabilization rate can be enhanced by modifying the chemical composition of the precursor fiber either by the use of an additive (comonomer), or by selective pretreatments such as impregnating with specific chemicals [see Leighton H. Peebles, “Carbon Fibers, Formation, Structure, and Properties,” CRC Press, pp. 7-26 and 128-35, 1994].
Early methods for stabilizing PAN fibers are taught by Houtz in U.S. Pat. Nos. 2,789,915, 2,913,802, 3,027,222 and 3,125,404, wherein oxidation of PAN under controlled temperature of 220-250° C. for several hours produced infusible material. The fibers, in general, acquired stability by an oxidation and cross-linking process. Oxidized PAN was converted commercially to carbon fibers in the early 1960s. Details are described by Tsunoda in U.S. Pat. No. 3,285,696. It was indicated in these patents that direct heating of PAN fibers to 1000° C. in a non-oxidizing atmosphere (nitrogen) produced a brittle, low-strength fiber product. However, with a prior pre-treatment step, a much stronger fiber was obtained when this material was subsequently processed to 1000° C. [see John Delmonte, “Technology of Carbon and Graphite Fiber Composites,” Van Nostrand Reinhold Co., New York, p. 55-61 and 190-1, 1981].
The technical literature indicates clearly that stabilization or oxidation treatment is the most critical processing step for determining the final properties of the manufactured carbon fibers. Economic estimates indicate that the stabilization step represents at least 20% of the total product cost and more than 30% of the total processing cost, and 70-85% of the total fiber processing time.
U.S. Pat. No. 3,699,210 teaches the use of laser sources for the carbonization and graphitization of PAN that had been fully oxidized at temperatures ranging from 180-500° C.
U.S. Pat. No. 3,914,394 teaches the use of ultrasonic waves in a liquid medium on fiber that had been pre-oxidized at temperatures below 400° C. After carbonization, the fibers had a higher strength and higher modulus of elasticity.
Many patents teach the treatment of the virgin and/or partially oxidized fiber with stabilization/oxidation promoting chemical agents. Early examples include U.S. Pat. Nos. 3,933,986; 3,820,951; 3,817,700; 3,814,577, 3,720,759; and 3,708,326. More recent examples include U.S. Pat. Nos. 6,733,737; 6,054,214; and 5,804,108.
U.S. Pat. No. 4,197,282 teaches the use of microwave energy only (rather than plasmas) to couple energy into pitch-based fibers (PAN based fibers do not couple microwave energy due to an extremely low value in the dielectric loss factor over a wide frequency range). The '282 patent therefore only applies to fibers that contain a sufficient quantity of carbon in the precursor to enable the efficient coupling of microwave energy into the fiber material. Furthermore, '282 is directed to the carbonization and graphitization of pitch-based fibers, not the oxidation or stabilization of thermoplastic materials such as PAN.
U.S. Pat. Nos. 5,412,246 and 5,330,935 are directed to forming a thin film on the surface of a semiconductor device and not to the carbon fiber production process.
U.S. Pat. No. 6,372,192 teaches the combined use of microwave energy and microwave based plasma for the carbonization and graphitization of carbon fiber. That work was independent of precursor material, but is limited to the carbonization and graphitization of materials that already have been substantially or fully oxidized/stabilized by some other means. The scope of '192 does not address the initial and broad oxidation/stabilization production stage.
U.S. Pat. No. 6,514,449 teaches the use of microwave energy and plasma to modify the surface topography of carbon fiber. The scope of '449 does not relate to the any of the oxidation/stabilization, carbonization or graphitization stages of carbon fiber production process. General discussions of fiber surface modification may be found in several references [Mittal, K. L. and Pizzi, A., “Adhesion Promotion Techniques. Technological Applications,” Marcel Dekker, pp. 67-76 and 139-73 (1999); D'Agostino, R., “Plasma Deposition, Treatment, and Etching of Polymers,” Academic Press, pp. 321-67 (1990); J. B. Donnet, T. K. Wang, S. Rebouillat and J. C. M. Peng, “Carbon Fibers,” Third Edition, Marcel Dekker, Inc., pp. 180-9 (1998)]. These teachings do not address stabilization or oxidation, but rather the modification of surface morphology or surface chemistry as a means of modifying interactions between fiber and matrix in a composite.
US Patent Application Publication No. 2003/0051993 A1 describes a “nonthermal capillary discharge plasma” device for activating various chemical reactions. Among other chemical processes, this patent application suggests the idea of applying the device to stabilize PAN fibers; however, the publication does not provide enabling details for a viable PAN stabilization process nor present evidence that the process has been successfully implemented.